Effect of composition on extreme anharmonicity of chemical bonds in Zr-doped ceria

Chemical bond anharmonicity, meaning that bonds do not behave like perfect springs, influences many important material properties, including thermal expansion, ionic conductivity, nonlinear optical behavior, and electrostriction, where an electric field causes a material to change shape. The widely used Uchino-Cross model assumes that this anharmonicity is small and predicts that electrostriction should increase with dielectric permittivity and elastic stiffness.

We studied the dielectric, mechanical, and electromechanical properties of zirconium-doped ceria (ZrₓCe₁₋ₓO₂₋δ) with different compositions and oxygen contents. For low zirconium concentrations (x ≤ 0.1), partially reoxidized samples showed a strong increase in dielectric permittivity and electrostriction as the Zr content increased. In some cases, the electrostriction coefficient became exceptionally large, indicating unusually strong bond anharmonicity. The effect was so pronounced that the anharmonic contribution exceeded the harmonic one, suggesting that the assumptions of the small-anharmonicity model may no longer be valid.

To understand the origin of this behavior, we examined the local atomic structure using EXAFS spectroscopy. The results revealed strong local disorder and highly anharmonic Zr-O bonds. Small amounts of trivalent dopants reduced electrostriction by limiting Zr displacement, while oxidation treatments largely removed the effect and restored the behavior of undoped ceria. This demonstrates that oxygen non-stoichiometry is essential for the observed response.

Density functional theory and molecular dynamics simulations further suggest that distorted Zr-centered oxygen environments create dynamic elastic dipoles that act collectively to produce giant electrostriction.

Overall, our experimental and theoretical results show that controlling the defect chemistry of zirconium-doped ceria can tune bond anharmonicity and significantly enhance the conversion between electrical and mechanical energy.